In a conventional solenoid type induction heating apparatus, although only a surface has a high temperature by a skin effect, a specified time is taken so that heat energy is sufficiently diffused into the inside of a plate and the temperature of the surface becomes lower than that at the center in plate thickness, and a temperature distribution in a plate thickness direction becomes appropriate.
For example, see JP-A-10-128424 (page 5, FIG. 1).
Further, in a transverse type induction heating apparatus, at the inlet side of a finish rolling mill, an inductor is moved in the width direction of a front edge part or a tail edge part of a material to be rolled so that the whole range of the material to be rolled is heated, and the inductor is moved to an edge part in the width direction of the material to be rolled so that the edge part in the width direction is continuously heated.
For example, see JP-A-1-321009 (page 3, FIG. 1).
In the conventional solenoid type induction heating apparatus, as a heating frequency becomes high, an induced current concentrate on the surface of the material to be rolled and flows, and the excessive temperature rise of the surface becomes large.
Besides, as the plate thickness becomes large, the excessive temperature rise of the surface with respect to the inside becomes large.
Thus, there has been a problem that it becomes necessary to take a sufficient time to make the temperature distribution in the plate thickness direction appropriate.
Further, in the transverse type, its object is to heat only the edge part of the material to be rolled in the plate width direction, the front edge part of the plate, and the tail edge part, and the inductor is moved to the center part in the plate width in order to heat the plate front edge part and the plate tail edge part in the plate width direction, and therefore, there has been a problem that the plate width center part of the material to be rolled can not be continuously heated in the longitudinal direction.